1. Field of the Invention
This invention relates to a vibration and acoustic wave detecting device employing a piezoelectric element. More particularly, this invention relates to an improved construction of a resonant-type vibration and acoustic wave detecting device employing a piezoelectric element which makes it possible to increase its available frequency bandwidth by lowering its quality factor Q.
2. Description of the Prior Art
With respect to a piezoelectric knock sensing device of a type employing a piezoelectric conversion element, a piezoelectric type knock sensor having a construction in which a resistor is connected in parallel with a piezoelectric conversion element so as to broaden its available frequency bandwidth is already disclosed by prior art references, for example, by Japanese Patent Unexamined Pub-ications Nos. 58-142227 and 58-142228.
As mentioned above, it is publicly known to make a resonant-type detector employing a piezoelectric element which has a construction in which a resistor is connected in parallel with the piezoelectric element to decrease its quality factor thereby to have a broad available frequency bandwidth, with a view to removing a defect such that the available frequency bandwidth becomes narrow as the quality factor increases. However, there is no prior art reference which has disclosed any concrete structure for incorporating a resistor in the resonant-type detector employing a piezoelectric element.